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molybdenum; Raman spectroscopy; silicates; structure–activity relationships; synthesis design
Abstract:
Silica-supported molybdenum oxide catalysts with high densities of disperse molybdenum oxide species of up to 3.5 atoms nm−2 were prepared by using a novel synthesis approach on the basis of functionalized mesoporous SBA-15. As revealed by using Raman spectroscopy and thermal analysis, the synthesis mechanism consists of an ion exchange of heptamolybdate ions (Mo7O246−) into the porous functionalized framework and decomposition of the intact precursor into smaller fragments during final calcination. The amount of dispersed molybdenum oxide as well as the ratio of dispersed to crystalline molybdenum oxide was determined in detail for Mo densities of 0–13.0 atoms nm−2 by using a combination of X-ray fluorescence (XRF), energy-dispersive X-ray analysis (EDX), and Raman spectroscopy. Up to 3.5 atoms nm−2 dehydrated molybdenum oxide is present exclusively as dispersed species. At higher densities, additional Mo is introduced as crystalline MoO3, and the amount of dispersed species remains unchanged. The structure of the supported molybdenum oxide under ambient conditions largely resembles that of heptamolybdate ions. Dehydration leads to the formation of grafted monomeric and connected molybdenum oxide surface species. Detailed X-ray photoelectron spectroscopy (XPS) analysis revealed an increase in the Mo/Si ratio as well as a positive binding energy shift upon dehydration, indicative of an increase in the dispersion of the supported molybdenum oxide. The combination of Raman and XPS analyses establishes a correlation of the changes in structure and dispersion of the supported molybdenum oxide species.
